LITHIUM IRON PHOSPHATE

BATTERY

(LiFePO4)

PREFACE

The field of batteries is one of the fastest growing and one of the most innovative field. With the size of devices getting smaller and smaller and the increasing number of functions that they are required to perform the battery has to have increasing power along with smaller in size. These requirements has lead to increased research in this field and the report is the study of one of those innovations, namely ‘Lithium Iron Phosphate Battery’.

ABSTRACT

THE lithium iron phosphate battery one of the latest development in the field of battery technology. It is part of the family of Li-ion batteries which till now generally use LiCoO2 electrode. The report includes detailed study of the LiFePO4 batteries, their composition, working (charging and discharging), physical and chemical characteristics and applications.

TABLE OF CONTENT

1. Preface

2. Abstract

3. Acknowledgement

4. History and Introduction

5. Synthesizing Processes

6. Battery Composition

7. Charging and Discharging

8. Advantages

9. Disadvantages and their Improvement

10. Applications

11. References

HISTORY and INTRODUCTION

The lithium iron phosphate battery electrode was first described by “JOHN GOODENOUGH’S” research group at the “UNIVERSITY OF TEXAS” in 1996 as the cathode material for rechargeable lithium batteries. LiFePO4 products are now in mass production and are used in industrial product by major corporations including Cessna, BAE Systems etc.

The lithium iron phosphate battery [LiFePO4] is a natural mineral of the “OLIVINE FAMILY”. It is the common minerals in the earth’s surface that is “magnesium iron silicate” [Mg,Fe]SiO4. The lithium iron phosphate battery is also called as “lithium ferrophosphate” battery ie. LFP and uses LiFePO4 as a cathode material. It is a type of rechargeable battery that offers low energy density but with longer lifetime, better power density (the rate that energy can be drawn from them) and are inherently safer in a lighter weight, more compact package. LiFePO4 battery is finding a number of roles in vehicle use and backup power.

Its key barrier to commercialization was intrinsically low electric conductivity. This problem however, is overcome by reducing the particle size and coating the lithium iron phosphate particles with conductive materials such as carbon and doping the result with materials such as aluminium, niobium and zirconium.

Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing. Thus, because of its low cost, non toxicity, high abundance of iron, excellent thermal stability, electrochemical performance and specific capacity, it has gained some market acceptance and is expected to widely expand the applications in the field of lithium batteries, and take it to the new fields such as electric bicycles, gas-electric hybrid vehicles and automation vehicles. However, a trade off is made for reduction of 14% in energy density, but higher energy variants are being explored.

ADVANTAGES

CLEAN: LiFePO4 batteries do not face any environmental issues unlike general Li-ion batteries which has cobalt has one of its constituents. Cobalt with its very nature of being carcinogenic is harmful for environment and humans.

LONG LASTING: A typical LiFePO4 battery lasts on an average 5 years longer than normal Li-ion batteries. LiFePO4

cells experience a slower rate of capacity loss ( hence greater calendar-life) than other lithium-ion battery chemistries such as LiCoO2

or LiMn2O4 lithium-ion polymer batteries or lithium-ion batteries. After one year on the shelf, a LiFePO4 cell typically has approximately the same energy density as a LiCoO2 Li-ion cell, because of LFP's slower decline of energy density. Thereafter, LiFePO4

likely has a higher density.

Some testing has shown that lithium iron phosphate batteries can last about 2,000 charge/discharge cycles, compared to perhaps 1,500 for lithium ion batteries

SAFE: One important advantage of LiFePO4 batteries over other lithium-ion batteries is their thermal and chemical stability, which improves battery safety. LiFePO4 is an intrinsically safer cathode material than LiCoO2 and manganese. The Fe - P - O bond is stronger than the Co - O bond, so that when abused, (short-circuited, overheated, etc.) the oxygen atoms are much harder to remove. This stabilization of the redox energies also helps fast ion migration. LiFepo4 has a lower +3 valence state in the charged cathode, this prevents O2 release and any possible thermal runaway. The Oxygen stays bonded therefore, nothing happens, even at temps at 160 celcius. Even in an overcharged

state, this type lithium battery is not combustable, no exothermic reactions takes place. Lithium iron phosphate cells are much harder to ignite in the event of mishandling especially during charge, although any battery, once fully charged, can only dissipate overcharge energy as heat. Therefore failure of the battery through misuse is still possible. It is commonly accepted that LiFePO4 battery does not decompose at high temperatures.

POWER: The cell provides constant power delivery within a tight voltage window over 80% of the state-of-charge (SOC) and storing the battery fully charged has minimal impact on its life. For an application with a narrow voltage window, this utilized energy can be maximized for the most efficient use. With their low impedance and thermally conductive

design, LiFePO4 cells can be continuously discharged at a fixed rate, a marked improvement over other rechargeable battery alternatives, and have consistent power over wide state of charge range.

The energy density (energy/volume) of a new LFP battery is some 14% lower than that of a new LiCoO2 battery.

In addition to this, LiFePO4 batteries are

capable of operation over wide range of temperatures which makes them suitable for use in

aircrafts. The discharging characteristics of the battery are shown in the corresponding graph over different temperatures. Also unlike general Li-ion (LiCoO2) batteries, the internal resistance of LiFePO4 batteries is low, remain stable over its age and charging cycles and infact reduces with increase in temperature as shown below.

DISADVANTAGES and THEIR IMPROVEMENT

Like all the things that exist in nature, LiFePO4 batteries also suffer from certain disadvantages that’s preventing their mass adoption, production and utilization.

These are:

Low Conductivity (10.10*10-9 S/cm) Low Diffusion constant of Lithium ions. The energy density (energy/volume) of a new LFP battery is some 14% lower than

that of a new LiCoO2 battery

The two former characteristics limits the charging and discharging rate of LiFePO4 batteries.

However these are now be reduced by the use of certain techniques like coating and replacing the material and converting the material into nano particles. Adding conducting particles in delithiated FePO4 raises its electron conductivity. For example, adding conducting particles with good diffusion capability like graphite and carbon to LiFePO4 powder significantly improves conductivity between particles, increases the efficiency of LiFePO4 and raises its reversible capacity up to 95% of the theoretical values.

Also, coating LFP with inorganic oxides can make LFP’s structure more stable and increase conductivity. Similarly, LiFePO4 with inorganic coating, such as ZnO and ZrO2, has a better cycling lifetime, larger capacity and better characteristics under the condition of a large discharge current. The addition of a conductive carbon in LiFePO4 increases the its efficiency. Mitsui Zosen Japan and Aleees reported that addition of other conducting metal particles, such as copper and silver, also increased LiFePO4’s efficiency. LiMPO4 with 1 wt. % of metal additives has a reversible capacity up to 140mAh/g and better characteristics under the condition of large discharge current.

APPLICATIONS

SOLAR LIGHTS: LFP cells are now used in some solar powered path lights because their

higher working voltage allows a single cell to drive an LED without needing a step-up circuit. Some models even claim to be 24 times more brighter than baseline path lights

VEHICLES:

o This battery is used in the electric cars made by Aptera and QUICC.

o Killacycle, the worlds fastest electric motorcycle, uses lithium iron phosphate

batteries.

o ZBoard electric skateboards use LFP batteries, offering ranges up to 20 miles.

o In May 2007, Lithium Technology Corp. announced a Lithium Iron Phosphate

battery with cells large enough for use in hybrid cars, claiming they are "the largest cells of their kind in the world.

o Used in aircrafts due to capability of operations over wide range of temperatures

and safety as compared to conventional Li-ion batteries.

ELECTRIC TOOLS: Many Electric tools and portable electric devices like toys, torches also now use these batteries due to their light weight, longer durability and safety.

BATTERIES IN CONSUMER ELECTRONICS: LiFePO4 batteries made

by BYD company of Shenzen, China, are being used in laptops made under the United

Nations ‘One Child One Laptop’ mission as they do not contain any toxic material.

BATTERY COMPOSITION

Similar to most other batteries LiFePO4 batteries have an outer metal case. The use of metal is particularly important here because the battery is pressurized. This metal case has some kind of pressure-sensitive vent hole whenever the battery gets so hot that it risks exploding from over-pressure, this vent releases the extra pressure while rendering the battery useless afterwards. The vent is strictly there as a safety measure. So is the Positive Temperature Coefficient (PTC) switch, a device that is supposed to keep the battery from overheating.

This metal case holds a long spiral comprising three thin sheets pressed together as shown in the corresponding figure:

Positive electrode Negative electrode Separator

Cathode (by weight):

90% C-LiFePO4 5% Carbon(superior graphite) 5% Polyvinylidene fluoride (PVDF) - Used as Separator, which separates the anode

and cathode physically while allowing the ions to pass through.

Electrolyte : A lithium salt in an organic solvent is used as electrolyte generally Ethylene carbonate-

Dimethyl carbonate (EC-DMC) & 1M solution LiClO4 is used as electrolyte

Pure lithium is very reactive. It reacts vigorously with water to form lithium hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes water from the battery pack.

Anode: Graphite or Hard Carbon with intercalated Metallic lithium.

CHARGING AND DISCHARGING

When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiFePO4 from the carbon.

The movement of these lithium ions happens at a fairly high voltage, so each cell produces 3.7 volts. This is much higher than the 1.5 volts typical of a normal AA alkaline cell that you buy at the supermarket and helps make lithium-ion batteries more compact in small devices like cell phones.

The separator sheet in LiFePO4 batteries keeps the electrodes apart and if this sheet gets punctured, the electrodes touch and the battery heats up very quickly. This heat causes the battery to vent the organic solvent used as an electrolyte, and the heat (or a nearby spark) can light it. Once that happens inside one of the cells, the heat of the fire cascades to the other cells and the whole pack goes up in flames. However it is rare and LiFePO4 is still one of the safest batteries around.

REFERENCES

Wikipedia (www.wikipedia.org)

How Stuff Works (www.howstuffworks.com)

Discovery Curiosity (curiosity.discovery.com)

A123 Systems (www.a123systems.com)